1. Field of the Invention
This invention relates to a method of correcting the density of a subtraction image obtained by subtraction processing of radiation images, and an apparatus for carrying out the method. This invention particularly relates to a method of correcting the density of a subtraction image obtained by digital subtraction processing of radiation images conducted by use of stimulable phosphor sheets so that background portions, soft tissue portions of an object, etc. in the subtraction image are reproduced always at a predetermined appropriate density, and an apparatus for carrying out the method.
2. Description of the Prior Art
Conventionally, a digital subtraction processing method is used for processing radiation images. In the method, two radiation images recorded under conditions different from each other are photoelectrically read out to obtain digital image signals, which are then subjected to subtraction processing with respect to the corresponding picture elements of the images, thereby to obtain a difference signal for forming an image of a specific structure contained in the radiation images. The method makes it possible to reproduce a radiation image of only the specific structure by use of the signal thus obtained.
Basically, subtraction processing is carried out by either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, the image of a specific structure is extracted by subtracting the image signal of a radiation image obtained without injection of contrast media from the image signal of a radiation image in which the image of the specific structure is enhanced by the injection of contrast media. In the latter method, an object is exposed to radiations having energy distributions different from each other to obtain two radiation images respectively containing the images of a specific structure recorded on the basis of the intrinsic radiation energy absorption characteristics of the specific structure. Then, the image signals of the two radiation images are weighted appropriately, when necessary, and subjected to subtraction to extract the image of the specific structure.
Since subtraction processing is extremely effective for diagnostic purposes in image processings for medical X-ray photographs, it has recently attracted much attention, and research has continued to develop improved method by use of electronic technology. The processing technique is particularly called the digital subtraction processing method, or normally, digital radiography (abbreviated as "DR").
A novel digital subtraction processing method has been proposed, for example, in Japanese Unexamined Patent Publication No. 58(1983)-163340 corresponding to U.S. Pat. No. 4,590,517. The method comprises the steps of (i) using two or more stimulable phosphor sheets exhibiting an extremely wide latitude of exposure to a radiation, (ii) exposing the stimulable phosphor sheets to the radiation passing through the same object under different conditions to have radiation images of the object stored on the stimulable phosphor sheets, image information of the specific structure being different between the radiation images, (ii) detecting the radiation images by scanning the stimulable phosphor sheets with stimulating rays to obtain digital image signals, and (iv) conducting a digital subtraction processing by use of the digital image signals. The stimulable phosphor sheets comprise a stimulable phosphor which is able to store a part of the radiation energy when exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, and then emits light in proportion to the stored energy of the radiation when exposed to stimulating rays such as visible light, as disclosed for example in U.S. Pat. No. 4,258,264. The stimulable phosphor sheets exhibit an extremely wide latitude of exposure and a markedly high resolving power. Therefore, when digital subtraction processing is conducted by use of the radiation images stored in the stimulable phosphor sheets, it is possible to obtain a radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy, regardless of the amount of the radiation to which the stimulable phosphor sheets are exposed.
In the aforesaid energy subtraction processing, when a high energy image signal detected from a stimulable phosphor sheet which was exposed to a high energy radiation passing through an object is designated by SH, and a low energy image signal detected from a stimulable phosphor sheet exposed to a low energy radiation is designated by SL, subtraction processing of the image signals is conducted between corresponding picture elements of the radiation images as expressed by EQU Ssub=a.multidot.SH-b.multidot.SL+c
where a and b denote weight factors and c deniotes a bias component for adjusting the density of the energy subtraction image formed by the difference signal Ssub to a predetermined value.
In the prior technique, the ratio b/a of the factor b to the factor a is experimentally determined based on the image recording method, radiation absorption characteristics of the image recording portion of the object, and the like, and the bias component c is also determined experimentally. However, since the appropriate value of the bias component c changes markedly with the image recording conditions and the object, the density range of the subtraction image becomes unsuitable for viewing, particularly for diagnostic purposes, and it becomes not always possible to diagnose correctly.